The present invention relates generally to a dome pressure maintaining valve and its utilization within a pilot operated pressure relief valve circuit. In another aspect, the dome pressure maintaining valve provides an apparatus and method for maintaining pressure in the dome of the relief valve thus keeping the valve in the closed position except under operation of the pilot.
Typically, pilot operated relief valve circuits utilize the pilot to control a large capacity, i.e., main pressure relief valve. Operation of the main valve is accomplished by controlled venting of a volume of fluid above the main valve member resulting in positioning the main valve closure member. In this way, the main relief valve will open due to the sensed over pressure or product pressure in excess of a predetermined value.
In a typical embodiment, the pilot valve operates a main valve by substantially venting the main valve head volume or dome volume to ambient pressure. The design of the main valve is such that prior to the relief valve venting, the main valve closure member or piston is in pressure equilibrium with the tank or pressure vessel or other supply source of pressurized fluids. When the head volume above the relief valve piston is vented to atmosphere, the tank pressure is utilized to lift the piston and open the main valve providing the desired pressure relief. An additional pressure signal to the pilot valve is produced, for example, by a ram tube located adjacent to the tank and/or pressure vessel, which provides an indication of true tank or product pressure allowing the pilot main valve to reset at a correct tank pressure independent of the magnitude of main valve flow.
In pilot operated relief valve circuits, relief valves having opening or relief characteristics of the proportional and instant opening, i.e., "pop" variety are utilized. Relief valves having a proportional opening characteristic provide venting or relief capacity which is proportional to the difference between a predetermined or set pressure and the particular pressure acting on the valve. In contrast, the instant opening or "pop" relief valves provide discharge characteristics which are a constant or maximum after sense pressure exceeds the predetermined or set value.
There have been developed a number of systems for monitoring and protecting flow lines and pressure vessels. These systems include high-low systems which are adapted to maintain flow through a flow line when the pressure within the lines is within a preselected range of pressures and close a line when the line pressure falls outside the selected range. These systems also include safety-relief systems which are adapted to vent protected flow lines or pressure vessels when the pressure exceeds a certain maximum. Examples of safety-relief valves generally include a relief valve which includes a dome and a valve element. The valve element has an effective area that is smaller than the effective area of the dome. Both the dome and the valve element are exposed to in-service pressure, which maintains the relief valve in a close position. A controlled system is provided which is adapted to supply in-service pressure to the dome when the in-service pressure is less than the maximum. However, when in-surface pressure exceeds the maximum, the controlled system is adapted to block in-service pressure and bleed the dome thereby allowing the relief valve to open.
Pilot operated pressure relief valves for liquid or gas service are available in various sizes and pressure ranges from, for example, 30 to about 1500 pounds per square inch. These relief valves are designed for use in a variety of installations wherein there is a requirement to exhaust the over pressure volume or gas. These valves are employed by oil and gas producers, petrochemical and chemical plants, pipe line companies, oil refineries, transportation systems and the like.
The relief valve is closed when below its set point. The normally open pilot allows the system pressure to enter the piston housing cavity or dome of the relief valve on top of the free-floating piston. The top of the relief valve piston has a larger area than the valve seat where the piston seals. Equal pressure at both ends of the piston creates a differential downward force which holds the piston tightly closed on the valve seat.
In a typical pilot operated relief valve system, when the system pressure reaches the pilot set point, the pilot piston forces the pilot stem upward by compressing the pilot valve spring. This movement of the stem simultaneously blocks the system pressure passageway through the pilot and commences the bleeding of pressure from the relief valve piston housing cavity or dome. Decreasing pressure on top of the relief valve piston allows system pressure to move the piston off the valve seat, discharging the system overpressure.
One problem has been recurring on a regular basis in the operation of pilot operated relief valves. At installation, the relief valve may be at the open position and it is difficult to achieve the closed position. It may take application of pressure in the system to build pressure on the dome to close the valve. Many times the open valve outruns the input of pressure and remains open. In the past, the user has been forced to remove the control line to the dome and apply pressure with a gas source in order to move the valve to the closed position. Once the valve is moved to the closed position, the system can be pressurized and the valve works properly Such a difficulty in processed startup at installation presents a problem to the user. In addition to the initial installation startup difficulties, problems have been encountered with the relief valves in a similar vein because the valve will open and remain open upon rapid application of pressure on restart after a system shutdown where pressure was reduced to zero.